Dynamic Equilibrium with straws
Setup:
Fill two beakers half-full with water and add a drop of food coloring to each cup to
improve visibility. Use two straws, one big and one smaller in diameter. Label the cups
‘A’ and ‘B’. Label the small straw ‘A’ and big straw ‘B’. Place straws according to cups.
Make sure that during exercise that the straws stay with their proper cup.
Class directions:
1)
Make sure straws are touching bottom of cups.
2)
Cover the top of straw ‘A’ with finger.
3)
Transfer fluid in straw ‘A’ to cup ‘B’
4)
Cover the top of straw ‘B’ with finger
5)
Transfer fluid in straw ‘B’ to cup ‘A’
6)
Repeat until dynamic equilibrium is reached.
Post-Lab Questions:
1)
2)
3)
4)
5)
6)
7)
8)
What do the diameter of straws represent?
What do water levels represent?
Why has water level stopped changing?
If straw ‘A’ doubled in size (or use ‘A’ twice per one ‘B’), then what can you
predict will happen?
If straw ‘B’ doubled in size, then what can you predict will happen?
Which cup is favored? What cup has more water? Why?
If ‘A’ is reactant and ‘B’ is product then what is the Keq?
Relative to ‘1’, what is Keq?
Answers:
1)
Rates of reaction
2)
Concentration of solute. (If students don’t draw the connection, mention that
the water is solute and that the entire cup is solution)
3)
Amount of water transferred between cups is now equal. Equilibrium is
reached
4)
Water levels would be closer together but ‘A’ would still be higher.
5)
Water levels would be further apart.
6)
‘A’ is favored because rate of reaction from ‘B’ to ‘A’ is greater than the
opposite.
7)
Keq is lesser than 1 because reactants are favored.